Density-Dependent Liquid Nitromethane Decomposition: Molecular Dynamics Simulations Based on ReaxFF

Abstract

The decomposition mechanism of hot liquid nitromethane at various compressions was studied using reactive force field (ReaxFF) molecular dynamics simulations. A competition between two different initial thermal decomposition schemes is observed, depending on compression. At low densities, unimolecular C–N bond cleavage is the dominant route, producing CH3 and NO2 fragments. As density and pressure rise approaching the Chapman–Jouget detonation conditions (∼30% compression, >2500 K) the dominant mechanism switches to the formation of the CH3NO fragment via H-transfer and/or N–O bond rupture. The change in the decomposition mechanism of hot liquid NM leads to a different kinetic and energetic behavior, as well as products distribution. The calculated density dependence of the enthalpy change correlates with the change in initial decomposition reaction mechanism. It can be used as a convenient and useful global parameter for the detection of reaction dynamics. Atomic averaged local diffusion coefficients are shown to be sensitive to the reactions dynamics, and can be used to distinguish between time periods where chemical reactions occur and diffusion-dominated, nonreactive time periods.

Thermal decomposition of solid phase nitromethane under various heating rates and target temperatures based on ab initio molecular dynamics simulations

“…Compared with the time evolution of potential energy on a unit cell in the inset of Fig. 4a, the potential energy curves on a supercell are similar and in accordance with previous simulations [13]. In addition, the final pressures on a supercell are −16 GPa, −14 GPa and −12 GPa at 2,400 K, 3,000 K and 4,000 K, respectively, which is also in excellent agreement with the results on a unit cell (Fig.…”

“…4b). And the pressure increments are also the same as in Rom's results [13], meaning that, regardless of simulation cell size, the time evolution of potential energy and pressure is similar in the fast annealing simulations, and dependent mainly on target temperature.…”

“…The other kind of first chemical reaction in our simulation is unimolecular C-N bond cleavage. Generally, the C-N bond is considered as the weakest bond and the one most likely to break first in gas phase and low density liquid phase, because its bond energy is only D 0 =60.1 kcal mol −1 [8,13]. And yet C-N bond cleavage as the first reaction in condensed phase nitromethane is rarely reported.…”

“…Meanwhile, the appearance of water was considered as the beginning of the exothermic chemistry in this latter work [12]. Rom et al [13] applied molecular dynamics with ReaxFF to probe the initial decomposition mechanisms of liquid nitromethane at various compressions, while Guo et al [14] investigated solid nitromethane impacted on a (010) surface. Competition mechanisms were observed both in initial decomposition stage of compressions and in shock detonation conditions.…”

Elucidation of the dynamics for hot-spot initiation at nonuniform interfaces of highly shocked materials

“…Thus ReaxFF provides the possibility of realistic simulations to probe the atomistic mechanism controlling detonation, [5][6][7][8][9][10][11][12] and leads to an accurate description of the complex chemistry of cyclotrimethylene trinitramine (RDX) under shock-loading conditions, 6 and similar calculations on polyethylene (PE) and poly(4-methyl-1-pentene) polymer lead to good agreement with the experimental Hugoniot. 9 It is generally accepted that detonation of PBXs is initiated at hot spots, but the mechanism responsible for hot-spot ignition is not clear.…”